The use of alumina for substrates used in separation and filtration is well known, and is one of the most commonly used inorganic materials for this purpose.
While it has often been suggested that titanium dioxide can also be used for fabricating substrates and membranes for separating and filtering purposes, it has not been generally possible or desirable to do so until the present invention.
One of the major problems with titania materials is their small particle sizes. Most commercial titania powders are used in the paint industry as pigments. These powders rarely have mean particle sizes greater than 3 micrometers.
Such small particle sizes are not suitable for making membrane supports for use in gas separations and microfiltration or ultrafiltration. Powders having small particle sizes generally produce substrates with small mean pore sizes, thus substantially reducing the flux through the porous body of the support.
Titania materials are more expensive than alumina materials, and would not provide a cost effective substitute were it not for the fact that membranes of titanium dioxide have a greater throughput during aqueous separations. (See A. Larbot, J-P. Fabre, C. Guizard, L. Cot and J. Gillot, "New Inorganic Ultrafiltration Membranes: Titania and Zirconia Membranes" J. of Am. Ceramic Soc., 72[2], 257-61, 1989.) Also, titania membranes and membrane. supports that can be properly fabricated are generally more durable for use with acidic and basic solutions, and particularly for basic solutions.
More recently, attention has been focused on the dangers of utilizing alumina substrates for processing water, foodstuffs and other comestibles, although it has not been conclusively determined that aluminum or alumina leaching into such ingestible commodities is harmful.
This fact, in addition to the other aforementioned advantages, has spurred increased interest in fabricating substrates and membranes from titanium dioxide materials.
The invention has been able to obtain titania powders of desirable particle size and distribution, and process them into efficacious substrates.
The invention also reflects the discovery that certain unacceptable powders when combined in admixture with other acceptable titania powders provide a synergistic extrudate having properties greater than either powder standing alone.
The invention also addresses a method of how to mix, shape or form, and sinter these powders to provide substrates useful for supports for microfilter, ultrafilter or gas separation membranes.